The present invention relates to a device for focusing a microscope objective on a sample, the device including a positioning unit having a main body, an objective holder movably supported on the main body and adapted to hold the microscope objective, and an actuator for moving the objective holder along the optical axis of the microscope objective.
In high-resolution light microscopy, it is particularly important that the microscope objective be held in a stable and precise position in the imaging beam path. For example, the spatial resolutions to be achieved in modern light microscopic methods are desired to be below 50 nm in a lateral direction; i.e., perpendicular to the optical axis of the microscope objective. Therefore, unintentional relative lateral movements between the sample and the microscope objective, which may result, for example, from temperature fluctuations, are to be avoided to the extent possible. This holds especially true for a number of methods which use imaging times of several minutes, during which even the smallest displacements of the microscope objective perpendicular to the optical axis would together result in image offset and, thus, in a correspondingly reduced lateral resolution.
In addition, in the light microscopic methods of the aforementioned type, the microscope objective must be focused on the sample with particularly high accuracy. The travel of the microscope objective along the optical axis is typically on the order of about 100 μm. Among the positioning units used in microscopes to cause the focusing movement of the microscope objective along the optical axis, increasing use is made of high-precision MEMS devices which include a main body, an objective holder movably supported on the main body, and an (for example, piezoceramic) actuator for precisely moving the objective holder, and thus microscope objective, along the optical axis so as to focus the microscope objective on the sample.
A conventional microscope objective is usually threaded at its rear end; i.e., the end facing away from the sample. This thread is used, for example, to screw the microscope objective into a commercially available objective turret. When using a positioning unit of the aforementioned type, this microscope thread is used to screw the microscope objective at its rear end into the objective holder, possibly with a suitable adapter positioned therebetween.
In the prior art, there are known various other approaches which aim to ensure that the microscope objective is held in as stable a position as possible and focused with high accuracy. For example, U.S. Patent Application No. 2002/0015225 describes a microscope in which the sample holder and the focusing unit are mounted directly on the microscope objective. However, this has the disadvantage that easy changing of microscope objectives is no longer possible. Moreover, manipulation of the sample has a direct effect on the focusing unit, which may impair the focusing accuracy.
European Patent Application EP 1 418 456 A1 describes a microscope in which unwanted deviations of the position of the microscope objective perpendicular to the optical axis are to be prevented by a rotationally symmetrical configuration around the optical axis and the use of optical components whose thermal expansion coefficients are suitably matched to each other.
International Patent Application published under the publication number WO 2006/056178 A1 discloses a microscope objective which is integrated into a sample stage by means of a guide sleeve. However, while this does increase the stability along the optical axis, there is a not negligible play of the microscope objective within the guide sleeve in a direction perpendicular to the optical axis.
U.S. Pat. No. 6,731,327 B1 describes a positioning unit for moving a microscope objective along the optical axis. This positioning unit has a clamping device that additionally stabilizes the microscope objective in its operating position. However, providing the clamping device makes this a relatively complex design.
From the German Patent Application published under the publication number DE 199 49 044 A1 there is known a device for fine positioning of a microscope objective using a double parallel spring element. The double parallel spring element acts on the end of the microscope objective that faces away from the sample, as a result of which the opposite free end of the microscope objective facing the sample may tilt to a not negligible degree, leading to unwanted image offset and a correspondingly reduced spatial resolution.
From the U.S. patent application published under the publication number US 2009/0284853 A1 a focusing device is known, comprising a pair of leaf springs, each of which is coupled with one end to a main body and with the other end to an objective holder. The objective holder is formed by an external thread arranged on the outer surface of the lens barrel and a nut engaged with the external thread.
In the U.S. patent application published under the publication number US 2010/0091363 A1 an objective holder is described, with which one of a plurality of objectives can be selectively brought into the microscope beam path. The plurality of objectives respectively have a contact surface arranged along the optical axis at the same height, which contacts a corresponding, fixed contact surface, so that parfocality between the various objectives is provided.